Reducing Sugars

Another characteristic of monosaccharides is that they can act as mild
reducing agents. This is because the aldehydo group that is present can be oxidized to
form a carboxylic acid group, or in the presence of a base, a carboxylate ion group.

Fructose can also act as a reducing sugar, even though it has a ketone group instead of
an aldehyde group. Under basic conditions, the fructose molecules can, essentially, have
the location of the carbonyl bond switched to convert them into a glucose molecule. This
occurs in a number of steps involving removing hydrogens from the #1-C and its oxygen and
moving them to the #2-C and its oxygen.

In one sense, monosaccharides that are in the ring form are not reducing
sugars because they don't have the aldehydo group that can be oxidized. However, because
they're in equilibrium with the open form, any monosaccharide molecule that's in a ring
form will, within a fraction of a second, be in the open form and, thus, be able to react
with the oxidizing agent and reduce it.

Lab Work

Your experiment for this week's lesson is to test a variety of carbohydrates to see
whether or not they are reducing sugars. In a sense, that test can be used to determine
the presence of a monosaccharide because a monosaccharide is a reducing sugar and,
therefore, presumably, if you have a reducing sugar, you have a monosaccharide. But for
reasons that you will see soon, that presumption is not entirely correct. If you wish, you
could do the experiment at this time, although I would recommend that you wait until after
we've talked about disaccharides and polysaccharides.